Metal detector having constant reactive transmit voltage applied to a transmit coil

ABSTRACT

An electronic metal detector having, a transmit coil arranged and adapted to transmit an alternating magnetic field associated with a reactive transmit coil voltage, the transmit coil being connected to transmit electronics arranged and adapted to generate a transmit signal, the transmit electronics having at least two power sources, a first power source and a second power source, wherein the first power source is adapted and arranged to connect to the transmit coil for at least a first period, and the second power source is adapted and arranged to connect to tie transmit coil for at least a second period, the said transmit electronics including at least one servo control negative feedback loop, a first servo control negative feedback loop, which is adapted and arranged to monitor a transmit coil current for at least part of the said first period, and to control at least part of the said transmit signal, the transmit electronics being adapted and arranged to control the transmit signal to produce the reactive transmit coil voltage to be approximately constant and approximately equal to zero while the transmit coil current is non-zero and approximately constant for at least part of the first period; and receive electronics which are adapted and arranged to receive and process a receive magnetic field during at least some of the first period to produce an indicator output.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/373,046, with a 371 (c) date of Jun. 2, 2009, and entitled“METAL DETECTOR HAVING CONSTANT REACTIVE TRANSMIT VOLTAGE APPLIED TO ATRANSMIT COIL,” which is incorporated here by reference in its entiretyfor all purposes.

TECHNICAL FIELD

This invention relates to a metal detector which transmits at least someperiods of constant reactive transmit coil voltage equal toapproximately zero whilst the transmit coil current is non-zero, and amethod of assisting accuracy of this constant reactive voltagegeneration.

This transmission has particular application to metal detectors whichare to operate in environments which may contain magnetic soils, are tobe insensitive to transmit induced magnetic signals from such soils,including effects of such soils modulating the inductance of thetransmit coil and also including varying soil magnetic coupling oftransmit coil with a receive coil, whilst the metal detector is toproduce an indicator output responsive to metal targets.

BACKGROUND

The general forms of most metal detectors which interrogate soils areeither hand-held battery operated units, conveyor mounted units, orvehicle mounted units.

Examples of hand-held products include detectors used to locate gold,explosive land-mines or ordnance, coins or treasure. Examples ofconveyor-mounted units include fine gold detectors in ore minimaoperations. An example of a vehicle mounted unit is a unit to search forland-mines. These units usually consist of a transmit coil which maytransmit an alternating magnetic field associated with a reactivetransmit coil voltage, transmit electronics which may generate atransmit signal applied to the transmit coil, and receive electronicswhich may receive a magnetic field and process received signals toproduce an indicator output. The received magnetic field may be detectedby a receive coil in most metal detectors, or the transmit coil in somemetal detectors such as pulse induction units. The most numerousproducts of the above examples are the hand-held, battery-operatedproducts. It is desirable that these have good buried target detectionrange, especially in magnetic soils which may contain ferrimagneticmaterials. Such metal detectors comprise receive electronics whichprocesses a received magnetic field such that the indicator output isresponsive to metal targets buried in such soils but not responsive themagnetic soils.

For state-of-the-art metal detectors metal target signals at the limitof the electronic noise produce voltage signals in a receive coil of theorder of ten nano Volts. In a metal detector with a “nulled” transmitand receive coil, the varying reactive voltage component from the highlymagnetic soils, found in most Australian goldfields for example, may betypically of the order of tens to hundreds of milliVolts across thereceive coil. Reactive components are usually symbolised as “X,”Resistive soil components, usually symbolised by “R,” are typically ofthe order of a hundred times less than the X component. The R soilcomponent is predictable and thus the R soil component may be nulled asdescribed in the patents in the table. As X is poorly correlated with Rin magnetic soils and thus unpredictable, the cancellation of theeffects of X is essential in order to accurately null magnetic soilsignals, X contamination in a resistive demodulated signal is a problemfor any metal detector system which transmits non-zero reactive voltagesduring resistive synchronous demodulation, such as for examplemulti-frequency sine-waves. One problem of X contamination of resistivesynchronous demodulation is non-linear behaviour of receive electronics.In order that this component is less than the smallest detectable metaltarget signals, the receive electronics must be accurately linear to anorder of 10⁶ to 10⁷. This is difficult and especially difficult at tensof kHz or higher. In order that any absolute “phase” inaccuracy of ametal detector system which transmit non-zero reactive voltages duringresistive synchronous demodulation does not significantly impede thecancellation of signals from magnetic soils, the absolute Xcontamination of the resistive components should be about two orders ofmagnitude less than the absolute resistive components. Hence, theabsolute accuracy of the “phase” needs to be of the order of 10⁴. Thistoo is difficult.

The transmit coil may be approximated as an effective inductivecomponent L, in series with an effective resistive component R, whichmay for mathematical convenience include resistance of cabling andconnectors and some elements of the transmit electronics. Thus thetransmit coil plus electronics effective resistance has the effectivetransmit coil time constant τ=L/R. As v=Ld(i)/dt where v is the transmitcoil reactive voltage, if the transmit coil current i is constant, thenv=0. Thus maintaining the transmit coil current constant for a periodresults in a zero transmit coil reactive voltage. The applied voltageacross the transmit coil is u=v+iR. Hence when the transmit coilreactive voltage is zero, the signal applied across the coil includingthe transmit coil cable resistance is iR. These equations assumetransmit coil cable and electronics with no stray capacitance orparallel resistance. For a given transmit alternating waveform repeatingsequence, and for a constant coupling between the transmit coil and thereceive coil, the receive coil voltage waveform will be of constantmagnitude and form if the transmit coil alternating reactive voltagewaveform v(t) is of constant magnitude and form. As i is proportional to1/L for a constant transmit coil reactive waveform v(t), the transmitcoil current waveform i(t) is modulated by 1/L and so too therefore isRi(t), and τ too. Thus the applied voltage waveform needs to be modifiedas the transmit coil inductance is modulated. A method of maintaining aconstant reactive transmit coil voltage waveform is described in WO2005/047932 A1.

Bi-polar transmitting CW systems such as sine-waves andrectangular-waves and examples given below of this invention have anadvantage over similar fundamental frequency and power consumption pulseinduction systems in that they have intrinsically more gain for signalfrom targets with long time constants, such as large gold nuggets orunexploded ordnance. For example the resistive component of many CWsystems asymptotically decreases as 1/(target time constant) assumingthe target has an effective principal time constant, whereas a pulseinduction system normally have a response proportional to 1/(target dineconstant)².

SUMMARY OF THE INVENTION

This invention addresses these problems by disclosing methods capable ofaccurately setting the transmit coil reactive voltage v to near zero.The advantage of having v=0 during demodulation is that the signalreceived by the receive electronics is mostly free of X and thus alsoavoids the significant non-linearities and absolute phase problemdescribed above. This invention discloses a method conducive to highpower efficiency, accurate transmit electronics which may generate atransmit signal applied to the transmit coil, which may transmit analternating magnetic field associated with a reactive transmit coilvoltage, such that the transmit signal of the transmit electronics iscontrolled to produce the reactive transmit coil voltage to beapproximately constant and approximately equal to zero whilst a transmitcoil current is non-zero and approximately constant for at least aperiod during which a magnetic field may be received and processed byreceive electronics to produce an indicator output which is relativelyinsensitive to receive signals from magnetic soils, and havingrelatively high sensitivity to relatively long time constant metaltargets compare to comparable pulse induction systems, with possiblyreduced sensitivity to environmental magnetic interference andimprovement over Australian patent 2006905485.

BRIEF DESCRIPTION OF THE DRAWINGS

To assist with the understanding of this invention, reference will nowbe made to the drawings:

FIG. 1 shows an example of a electronic system capable of producing anapproximately constant reactive transmit coil voltage which isapproximately equal to zero whilst a transmit coil current is non-zeroand approximately constant for at least a first period, and independentof modulation of transmit coil inductance by magnetic soils.

FIG. 2 is an example of possible transmit and receive waveforms for FIG.1; this example being an approximate transmit coil current square-wave.

FIG. 3 is an example of an electronics system with more detail than FIG.1.

FIG. 4 is another example of a preferred embodiment of this invention.

FIG. 5 is an example of possible transmit and receive waveforms for FIG.4.

DETAILED DESCRIPTION

U.S. Pat. Nos. 4,942,360; 5,576,624; 5,537,041; 6,636,044; 6,653,838;and Australian patent 2006905485 describe various means of cancellingsignals arising from magnetic soils. This invention is an improvement onthese and WO 2005/047932. The table below lists these patents, themethod disclosed for each, the relative problems of each method and therelative advantage of this invention.

Pat. Disclosure Problems New advantage 4,942,360 Simultaneous multi-Difficulty in linear Eliminates linear frequency with selectedelectronic demodulation problem receive frequency demodulation.Difficult during reception. Phase component linear to set absolute phasereference problem combinations reference effectively eliminated5,576,624 Pulse induction with Overcomes above Higher sensitivity toselected receive time problems but relative slow time constant domaincomponent insensitivity to slow targets and more power linearcombinations time constant targets. efficient Poor power efficiency5,537,041 Multi-period Difficulty in linear Eliminates linearrectangular pulses with electronic demodulation problem selected receivetime demodulation. Difficult during reception. Phase domain component toset absolute phase reference problem linear combinations referenceeffectively eliminated. 6,636,044 Improved signal Applies to thisinvention processing signal-to- noise 6,653,838 Multi-voltage pulseImproved signal-to- See 5,576,624 induction noise version of 5,576,624with same problems 2005/ Constant transmit coil Improved transmission047932 reactive voltage when method suitable for transmit coil currentis efficient power non-zero consumption and accurate maintenance ofconstant reactive voltage 2006 Improved method for Applies to thisinvention 905485 magnetic soil signal suppression

In FIG. 1, transmit coil 1 approximately may be represented by aneffective resistive component 3 in series with an effective inductivecomponent 2. This is connected to transmit electronics 4, 5 a, 5 b, 6 a,6 b, 9 a, 9 b, 16, 17, 19 and 20 which may generate a transmit signalapplied to the transmit coil 1. A first power source 5 a, connected tothe system common ground 4, is connected to switch 21, A third powersource 5 b, connected to the system common ground 4, is connected toswitch 22. Switch 21 and 22 are controlled to be “on” or “off” by timingcontrol electronics 16 via control lines 17. Switch 25 and switch 26 areconnected to the system common ground 4 via resistor 8, to switch 21 andswitch 22 respectively, to the transmit coil 1, and to switch 23 andswitch 24 respectively. Switch 23 is connected to a second power source6 a which is connected to the system common ground 4, and switch 24 isconnected to a fourth power source 6 b which is also connected to thesystem common ground 4. Switch 25, switch 26, switch 23, and switch 24are controlled to be “on” or “off” by timing control electronics 16 viacontrol lines 17. Thus switch 21 connects or “switches” the transmitcoil to the first power source 5 a, switch 23 switches the transmit coilto the second power source 6 a, switch 22 switches the transmit coil tothe third power source 5 b, switch 24 switches the transmit coil to thefourth power source 6 b, switch 2.5 and switch 26 switch the transmitcoil to the system common ground 4 via resistor 8. These switches 21,22, 23, 24, 25, and 26 and resistor 8 may be said to form switchingelectronics 20. Resistor 8 is of low resistive value and used fortransmit coil current sensing by feeding a voltage across resistor 8 toan input of a first servo loop amplifier 9 a, and a third servo loopamplifier 9 b and a fourth servo loop amplifier 19. An output of thefirst servo loop amplifier 9 a is connected to a control input of thefirst power source 5 a, such that the first power source 5 a, resistor8, first servo loop amplifier 9 a and the said switching electronics 20form a first servo control negative feedback loop which controls currentflowing through resistor 8 to be approximately constant at least duringa first period. The outputs of the second power source 6 a and thefourth power source 6 b are constant voltages, which may be maintainedby a shunt regulator or local servo control negative feedback loops. Anoutput of the third servo loop amplifier 9 b is connected to a controlinput of the third power source 5 b, such that the third power source 5b, resistor 8, third servo loop amplifier 9 b and the said switchingelectronics 2.0 form a third servo control negative feedback loop whichcontrols current flowing through resistor 8 to be approximately constantat least during a third period.

Only one of switch 23, switch 25 and switch 21 is closed at any instantand only one of switch 24, switch 26 and switch 22 is closed at anyinstant. During the first period either switch 25 and switch 22 areclosed, or switch 26 and switch 21 are closed. During the second periodeither, switch 23 and switch 26 or 22 are closed, or, switch 24 andswitch 25 or 21 are closed.

Receive coil 29 receives a magnetic field via receive inductor 12 underthe influence of an alternating magnetic field generated by the transmitcoil 1. Receive coil 29 is connected to an amplifier 13 whose output isconnected to synchronous demodulators 14 which are controlled by thetiming electronics 16 via control lines 18. An output of synchronousdemodulators 14 is connected to further receive electronics processing15 which includes an indicator output 10. Amplifier 13, synchronousdemodulator 14 and further receive electronics processing 15 may be saidto form receive electronics 11. Transmit coil 1 and receive coil 12 maybe said to form a metal detector sensing coil 19. The synchronousdemodulation may include similar principles of U.S. Pat. No. 6,636,044.

The first power source 5 a and third power source 5 b may be combinedinto a single power source, a first combined power source, wherein itsoutput is controlled to produce an output when switch 21 is dosed duringa first period at a voltage controlled by an output of the first servoloop amplifier 9 a, and when switch 22 is closed during a third period,the output voltage of the combined first power source is controlled byan output of the third servo loop amplifier 9 b. The second power source6 a and fourth power source 6 b may be combined into a single powersource, a second combined power source, wherein its output may becontrolled by an output of a single servo loop amplifier which monitorsa voltage output of the second combined power source to form a servocontrol negative feedback loop to maintain the output of the secondcombined power source at an approximate constant voltage. However, thesecond combined power source may consist of a simple power supplyregulated with a shunt constant voltage element, such as a zener diodefor example, and have no need of any servo control negative feedbackloop to maintain the output voltage.

The transmit electronics in FIG. 1 may generate a transmit signal which,when applied to the transmit coil 1, may transmit for example a bipolaralternating magnetic field containing at least a first period with areactive transmit coil voltage to be approximately constant andapproximately equal to zero whilst a transmit coil current is non-zero.

To assist with the understanding of the operation of the FIG. 1, FIG. 2shows an example of possible signal waveforms. A time axis 50 is shownin each graph. FIG. 2 is an example of a transmit waveform symmetricboth in time, and polarity, which in this example, produces anapproximate transmit coil current square-wave 51, 52, 53 and 54.Waveform 40, 41, 43 and 44 shows the reactive transmit coil voltage, Afirst period commencing at the transition between 43 and 40 andterminating at time 56, the transition between 40 and 44, is a periodwith zero reactive transmit coil voltage 40 whilst a non-zero constanttransmit coil current 51 flows. Similarly in reversed transmit coilcurrent flow sense compared to the first period, a third periodcommencing at the transition between 44 and 41 and terminating at time55, the transition between 41 and 43, is another period with zeroreactive transmit coil voltage 41 whilst a non-zero constant transmitcoil current 52 flows. During the first period, the first power source 5a is applied to the transmit coil, with for example switch 21 closed andswitch 23 and switch 25 open. The opposite end of the transmit coil isconnected to the system ground 4 via resistor 8 and via switch 26 beingon, and switch 24 and switch 22 being off. The actual voltage appliedacross the transmit coil 45 during the first period is approximately thereactive transmit coil voltage 40 plus a voltage resulting from thetransmit coil current flowing through the transmit coil effectiveresistance 3. During a second period between time 56 and the transitionbetween 44 and 41, the transmit coil is switched to the second powersource 6 a. This is shown as 44 in FIG. 2 and is reduced in scale forclarity. During the second period, the reactive voltage across thetransmit coil is roughly the same as the actual voltage across thetransmit coil, assuming that this voltage is much higher than thevoltage applied across the transmit coil 45 during the first period.During this second period, switch 23 is closed and switch 21 and switch25 are open. The opposite end of the coil may be switched to either thesystem ground 4 via resistor 8, with switch 26 closed and switch 22 andswitch 24 open, or to the third power source 5 b with switch 22 closedand switch 26 and switch 24 open. Alternatively, during the secondperiod, this end of the coil may be switched to the third power sourcefor some of the second period and system ground for a different periodwithin the second period. The transmit coil is switched to the thirdpower source 5 b in a reverse polarity sense 46 compared to 45 (whenswitched to the first power source) during a third period. During thethird period, the third power source 5 b is applied to the transmitcoil, with for example switch 22 closed and switch 24 and switch 26open. The opposite end of the transmit coil is connected to the systemground 4 via resistor 8 and via switch 25 being on, and switch 23 andswitch 21 being off. During a fourth period between time 55 and thetransition between 43 and 40, the transmit coil is switched to thefourth power source 6 b in a reverse polarity sense to the secondperiod. This is shown as 43 in FIG. 2 and is reduced in scale forclarity. During the fourth period, the reactive voltage across thetransmit coil is roughly the same as the actual voltage across thetransmit coil assuming this voltage is much higher than the voltageapplied across the transmit coil 46 during the third period. During thisfourth period, switch 24 is closed and switch 22 and switch 26 are open.The opposite end of the coil may be switched to either the system ground4 via resistor 8, with switch 25 closed and switch 21 and switch 23open, or to the first power source 5 a with switch 21 closed and switch25 and switch 23 open. Alternatively, during the fourth period, this endof the coil may be switched to the first power source for some of thefourth period and system ground for a different period within the fourthperiod. The transmit coil approximately square-wave has transitions 54and 53 occurring during the second and fourth periods when the transmitcoil is connected to the second power source 6 a and the fourth powersource 6 b. Fine details of the waveforms shown in FIG. 2 are not shownsuch as low level transmit coil current ringing associated withswitching clue to transmit coil and electronics capacitances andinductances, and such as switching “glitches.” The slew rate duringcurrent changes 54 and 53 is approximately proportional to a voltage ofthe said second power source switched to the transmit coil during thesecond period and a voltage of the said fourth power source switched tothe transmit coil during the fourth period respectively. In FIG. 2received signal decays 71 and 72 from magnetic soils are shown, as toois a soil signal 73 and 74 during the second and fourth periods.

In FIG. 2, the currents of 51 and 52 are assumed to be of the sameabsolute magnitude. However, unless they are controlled to be so, thereis nothing to constrain an arbitrary offset current which might drift.If, say, the duration of the second period is fixed, and the duration ofthe fourth period may be controlled, and if a difference in valuebetween the transmit coil current between the first period and the thirdperiod is monitored by a fourth servo-loop amplifier 19 which bycontrolling the ratio of the fourth period to the second period durationcontrols the ratio of energy transferred to the transmit coil during thesecond period and the fourth period. The fourth servo-loop amplifier 19,timing control electronics 16, control lines 17, switching electronics20 and power sources form a fourth servo control negative feedback loopwhich controls the said difference in absolute values between thetransmit coil current during the first period and the during the thirdperiod to be approximately zero.

The example waveforms shown in FIG. 2 is a useful waveform and may bethought of as “pulse induction like,” but with finite transmit currentflow during suitable periods of resistive synchronous demodulation,namely the first and third periods.

For direct comparison of the waveforms in FIG. 2 with a similar pulseinduction system, assume for the pulse induction system that the powerconsumed in the transmit coil is the same for the waveforms in FIG. 2and the pulse induction system. Assume that the pulse induction “on”time is during the first period with −V volts (e.g. −5V) applied to thetransmit coil, the back-emf occurs approximately during the secondperiod (e.g. +200V), and the zero transmit coil current during thereceive third and fourth period. Assume ideal component behaviour withthe transmit coil current approximately a linear ramp, with the reactivetransmit coil voltage during the first period assumed to beapproximately constant, and back-emf period assumed to be of “zero”duration. Assume the first, second, third and fourth period form arepeating sequence.

As described in U.S. Pat. Nos. 5,576,624, 5,537,041, and 6,636,044, thetotal integrated synchronous demodulation linear combination must equalzero in order to cancel out environmental static magnetic fields and 1/felectronic noise. As this must occur in just the third and fourth periodof a pulse induction system compared to both the first and third periodwith the advantage of opposite polarity in the transmit coil currentsquare-wave system, less long time constant target attenuation occursfrom the latter system compared to the former system. In addition, asthere are 2 periods of high voltage in the transmit coil currentsquare-wave system compared to the phase induction system, the resistivesignal from fast time constant targets is roughly improved by 4/SQRT(6)for the same power consumed in the transmit coil. Hence the targetresponse is generally improved. In addition, if the synchronousdemodulation gain profile is symmetric in the transmit coil currentsquare-wave system, then even noise harmonics of the fundamentalfrequency are cancelled thus offering more immunity to environmentalnoise.

A bi-polar pulse induction system does not fare significantly better.

Assuming that the losses in the switching electronics are small and thepower sources efficient (e.g. switch mode power supplies withsynchronous rectifiers) and the voltage drop across any series linearregulator in the first power source is small (few 1/10^(th) of a Volt),this transmit coil square-wave current system must be nearly optimal intarget signal-to-noise ratio for a large range of time constant targetsfor a given transmit power dissipation and a system suitable forcancelling out magnetic soil signals with resistive synchronousoccurring during periods of zero transmit coil reactive voltage.

The signal from magnetic soil components during the third period from asingle second period (44) followed by the said third period isproportional to 1/t assuming the second period is effectively of “zero”duration, and assuming the soils have constant valued resistivecomponents to very high frequencies.

For a continuous repeating sequence of FIG. 2, the signal isapproximately proportional to β(t/T) where β(t)={ψ[(t+1)/2]−ψ[t/2]}/2,ψ(t)=d ln(Γ(t))/dt, and where T equals the duration of each of the firstand third periods.

FIG. 3 shows an expanded block diagram of FIG. 1. Many of the elementsare labelled the same as in FIG. 1 as these perform the same function.Two extra switches 82 and 83 are included in the switching electronicsfor practical reasons, mostly because of the parallel diodes in HETswitches 25 and 26, assuming these are FETs. Switch 82 and switch 83 arealso controlled by the timing electronics 16 via control lines 17.Control lines 98 control the operations of servo-loop amplifiers 9 a, 9b, and 7.

A power supply 80 (e.g. a switch mode power supply) in the first powersource 5 may have an output which varies slightly as the transmit coilcurrent changes. Hence for practical reasons, the first power source 5should include electronic circuitry whose output is of low impedance andwhose output voltage switched to the transmit coil during the firstperiod and third period may change at a substantially faster rate thanany variation in the output of power supply 80. This may be implementedby the first power source 5 including a series linear regulator 92 (suchas a series FET plus operational amplifier for example) whose outputvoltage is controlled by an output of the servo-loop amplifier 9 aduring the first period when switches 21, 26 and 82 are closed, and anoutput of the servo-loop amplifier 9 b during the third period whenswitch 22, 25 and 83 are closed, where the output of the series linearregulator 92 connected to switch 21 and switch 22 may be tightly voltagecontrolled via a fast local negative feedback loop formed using the saidoperational amplifier for example. Hence the first power source 5 of lowimpedance has an output connected to the switching electronics which mayinclude a series linear regulator 92 including a local negative feedbackloop.

The voltage of the second power source 6 is shown as 43 and 44 (secondand fourth periods) with a change in scale assuming, that this voltageis relatively high (e.g. 200V). Not shown is fine waveform detail, inparticular, assuming the value of storage capacitor 90 is relatively lowvalue (e.g. 2.2 microFarad), the peaks of the waveforms 43 and 44 shouldshow roughly small parabolic waveforms as the capacitor initiallycharges then discharges. Assuming that the second power source 6contains a switch mode power supply 84, an output rectifying diode 85 isshown. The voltage across capacitor 90 is controlled by servo-loopamplifier 7 controlling power supply 84. The servo loop amplifiermonitors the voltage across capacitor 90 via connection 91.

The receive electronics 11 processors a receive signal from the saidreceived magnetic field such that the indicator output 10 is responsiveto metal targets but not receive signals from magnetic soils which mayinclude ferrimatmetic materials in accordance with similar principlesdisclosed in U.S. Pat. No. 5,576,624. Resistive receive signals may begenerated by receive synchronous demodulation during the first and thirdperiods as the transmit reactive voltage is approximately zero.

As transmit electronics which generates a transmit signal repeatingsequence applied to the transmit coil as the electronics is not perfect,in order to reduce any inaccuracies in the waveform, each “independent”variable should be controlled with different servo control negativefeedback loops.

In order to accurately ensure the reactive transmit coil voltage to beconstant and approximately equal to zero whilst a transmit coil currentis non-zero and constant for the receive resistive synchronousdemodulation periods, whilst rapidly tracking changes in the inductanceof the transmit coil, advantage may be gained by the use ofsample-and-hold elements, for example, at the output of servo loopamplifiers which may be controlled by the timing electronics 16 viacontrol lines 98. Thus the transmit electronics may include at least onesample-and-hold element which feeds a first output signal to a controlinput of at least the first power source which controls the output ofthe first power source 5 during at least the first period, such that thesaid first output signal voltage is held constant during the firstperiod. Similarly a sample-and-hold element in the output of the thirdservo loop amplifier may control the constant output of the first powersource during the third period.

Suppose the transmit waveform consists of a repeating sequence of say afirst period where a first constant low voltage (from the first powersource, e.g. 45 in FIG. 2 of say u=+1 Volt) is applied to the transmitcoil of a first polarity, followed by a second high voltage during asecond period of short duration (say Ts) and of a second polarity (fromthe second power source, e.g. 44 in FIG. 2 of say V=−200 Volt), followedby third period where a third constant low voltage (from the first powersource, e.g. 46 in FIG. 2 of say u=−1 Volt) is applied to the transmitcoil of second polarity, followed by a fourth high voltage during afourth period of short duration (say Ts) and of a first polarity (fromthe second power source, e.g. 43 in FIG. 3 of say +V=+200 Volt), andthen this sequence repeated. If the average transmit current for eachcomplete sequence is zero, and the transmit coil current is constantduring the first period (e.g, say 51 in FIG. 2 of value +I) and thirdperiod (e.g. 52 in FIG. 2 of value −I) which results in a constant zeroreactive transmit coil voltage (e.g. 40 and 41 in FIG. 2), then as thevoltage applied across the coil during the second and fourth periods(−/+200V) is high compared to any voltage drop across effectiveresistance component impedance 3 (<=|1V|), the reactive transmit coilvoltage approximately equals the voltage across the transmit coil 1.Thus when this relatively high voltage is applied across the transmitcoil during the second or fourth period, the instantaneous reactivetransmit coil voltage is approximately defined and independent of theinstantaneous effective inductive component impedance 2 which may beunder the influence of magnetic soils and thus altering as the coilmoves relative to the magnetic soils. Hence it is useful if the voltageof the second power source is more than a factor of 10 that of thevoltage of the first power source. The change in transmit current duringthe fourth period is ΔI=+VTs/L=+I−−I=2I, where L is the transmit coilinductance which may vary as the coil moves relative to magnetic soils.As L may vary, so too will ΔI assuming Ts and V are constant, and henceso too does I vary in proportion to 1/L. Thus u must also vary inproportion to 1/L as the voltage across the transmit coil u=IR if thereactive voltage is zero, where R is the effective resistive componentimpedance 3. A similar explanation applies to the second period.

The response time of a transmit electronics system combination includingall the servo control negative feedback loops, must operatesignificantly faster, that is operate up to an effective higherfrequency than an information frequency bandwidth of the indicatoroutput, that is in essence the effective frequency information bandwidthat the output of resistive signals including low pass filtering that ispassed to an indicator output. This is because the response of the servoloops to correct say an inductance of the transmit coil altered by avarying proximity of the transmit coil to magnetic soils must rapidlytrack such changes so that the effect of these changes does notsignificantly contribute to an indicator output signal.

Unlike a pulse induction system, this transmit coil current square-wavesystem transmits a magnetic field whilst receiving resistive components,and these synchronous transmitted finite magnetic fields may induce arate of change of flux signal as the coupling between the transmit coiltransmitted alternating magnetic field and a magnetic field received andprocessed by the receive electronics alters when for example, thecoupling is modulated by the varying interrogation of magnetic soils.This component rate of change of receive resistive signal needs to becancelled to stop it contaminating an indicator output signal. Areactive receive signal may be formed by synchronously demodulating areceive signal during a period when the reactive transmit coil voltageis non-zero, for example during the second period and fourth period. Toeffect this said cancellation, the reactive receive signal is furtherprocessed by the receive electronics to be approximately differentiatedto give a differentiated reactive receive signal, and a linearcombination of the differentiated reactive receive signal and theresistive receive signal is selected so that a component of the saidresistive receive signal that is responsive to magnetically permeablematerial rate of change of coupling between the transmit coiltransmitted alternating magnetic field and a magnetic field received andprocessed by the receive electronics is approximately cancelled, and theresultant linear combination is further processed by the receiveelectronics to give an indicator output.

FIG. 5 shows another example of transmit waveforms which includes athird period and a first period of zero reactive transmit coil voltage,58 and 61, whilst a non-zero, constant transmit coil current flows, 64and 66, which may be implemented in FIG. 4. This includes the switchingelectronics 20 in FIG. 4 switching a fifth power source 93 to thetransmit coil via switch 94 and via switch 21 or switch 22, and switch82 during a fifth period, such that the transmit coil current 65 changessign during the fifth period when an output voltage 63 of the fifthpower source 93 (e.g. −5 Volt) switched to the transmit coil during thefifth period is greater in absolute magnitude to a voltage of the firstpower source switched to the transmit coil during the first period (e.g.1 Volt), and less than an absolute magnitude of a voltage of the secondpower source switched to the transmit coil during the second period(e.g. 200 Volt). A switch 97 is used to control the periods for whichthe outputs of the servo-loop amplifiers 9 a and 9 b control the outputvoltage of the series linear regulator 92. The fifth power source 93 isconnected to the system ground 4. FIG. 5 shows the voltage 60 applied tothe transmit coil for the first period, and the voltage 62 applied tothe transmit coil for the third period, and the voltage 57 applied tothe transmit coil for the second period. The fifth period is definedbetween times 76 and 77. The transmit coil reactive voltage 59 of thetransmit coil during the fifth period is proportional to exp(−t/τ). Thetransmit coil current during the fifth period is I₀exp(−t/τ)+V/R(1−exp(t/τ)) where V is the output voltage of power source93 switched to the transmit coil and to is the initial current of thefifth period.

The receive voltage signal 68 across receive coil 12 from magnetic soilsfrom a single fifth period pulse during the first period isapproximately proportional to Ei[t/T]−Ei[(t+1)/T]}exp[(−t+T)/τ] where Eiis the exponential integral across the receive inductor 12, where T isthe duration of the fifth period. The actual receive signal from arepeated transmit waveform sequence also includes the decaying signalsfrom previous transmissions.

The advantage of such a waveform wherein an output voltage of the fifthpower supply switched to the transmit coil during the fifth period isgreater in absolute magnitude to a voltage of the first power sourceswitched to the transmit coil during the first period, and less that anabsolute magnitude of a voltage of the second power source switched tothe transmit coil during the second period, such as −5 Volts, is thatthe receive waveform form magnetic soils is very different between thefirst 68 and third 69 periods, thus potentially improving the capabilityof discriminating between metal targets and magnetic soils. Also, asdescribed in U.S. Pat. No. 5,576,624 and U.S. Pat. No. 5,537,041,ferrous metal targets may be relatively well discriminated fromnon-ferrous targets if a reactive signal is derived from synchronouslydemodulating during the latter half of the fifth period. The signal 78from magnetic soils during period 5 is shown in FIG. 5.

A servo control negative feedback loop is required for each“independent” variable of the transmit signal repeating sequence forbest control of the transmit coil reactive voltage during the first andthird periods. Hence for example, for the transmission of the waveformshown in FIG. 5, assuming that the voltage and duration of the secondperiod, and the voltage at the output of the fifth power source arefixed, then the following variables may be controlled by individualservo loops: the voltage 60 of the first power source 5 applied to thetransmit coil during the first period, the voltage 62 of the first powersource 5 applied to the transmit coil during the third period, and theduration of the fifth period when the fifth power source is switched tothe transmit coil.

A fifth servo loop amplifier 96 monitors a difference in transmit coilabsolute current between the first period and the third period. Anoutput of the fifth servo loop amplifier 96 is connected to a controlledduration timer 95 which includes an input selector. An output of thecontrolled duration timer 95 controls switch 94 to be on during thefifth period. The controlled duration timer 95, fifth servo loopamplifier 96, switch 94, switching electronics 20, and fifth powersupply 93 form a fifth servo control negative feedback loop whichmaintains the difference in transmit coil current between the firstperiod and the third period to be approximately zero.

A second servo loop amplifier 99 monitors the output voltage of thesecond power source which includes a first capacitor 90 which is chargedonly by energy from the back emf of the transmit coil when the transmitcoil is switched to the said first capacitor 90 during the secondperiod. An output of the fifth servo loop amplifier 96 is connected to acontrolled duration timer 95 which includes an input selector. An outputof the controlled duration timer 95 controls switch 94 to be on duringsixth period between 32 and 75 in FIG. 5. During the sixth period, thefifth power supply is switched to the coil as shown by the appliedtransmit coil voltage 31, and the transmit coil current 33 increasesabsolutely. The controlled duration timer 95, second servo loopamplifier 99, switch 94, switching electronics, and fifth power supply93 form a second servo control negative feedback loop. The increase inenergy stored in the transmit coil resulting from the said increase intransmit coil current during the sixth period is discharged during partof at least the second period into the said first capacitor such thatenergy lost during the second period in the transmit coil and also inthe switching electronics associated with switching the transmit coil tothe first capacitor 90, is compensated by the said increase in energystored in the transmit coil resulting from the said increase in transmitcoil current. The second servo control negative feedback acts tomaintain the voltage across capacitor 90 constant. The power supplyconnected to the transmit coil during the sixth period may be adifferent power supply to the fifth. The transmit coil reactive voltageduring the sixth period is shown as 30 in FIG. 5.

Hence the transmit electronics is arranged and adapted to comprise atleast different servo control negative feedback loops to control thetransmit waveform such that reactive transmit coil voltage is controlledto be constant and approximately equal to zero whilst a transmit coilcurrent is non-zero during periods when the receive electronics isarranged and adapted to synchronous demodulate receive resistivecomponents, such as during the first and third periods for example.

Varying the duration of the fifth period when the fifth power source isswitched to the transmit coil is not ideal, but in practice powerefficient, inexpensive and gives satisfactory performance, A bettersolution for this situation is disclosed in WO 2005/047932, but this ismore expensive and less power efficient. If the repeated transmit signalvoltage waveform is that given in FIG. 5 and the voltages 57, 62, 60 arethe same as 43, 45 and 46 respectively as above, the change in currentΔI during the fifth period must equal the change in current during thesecond period and thus is also proportional to 1/L.

In an alternative preferred embodiment power source 5 may be acontrolled current source, but a system employing such a system exhibitsring transients from the transmit coil associated resonance.

Note that the roles of various servo control negative feedback loop maybe altered, which form combined servo-loop systems.

What is claimed is:
 1. A method for detecting a target in a soil, themethod including the steps of: generating a transmit signal, thetransmit signal includes a repeating sequence, each sequence includes afirst period and a second period, the first period preceding the secondperiod; transmitting a transmit magnetic field by applying the transmitsignal to a transmit coil; sensing a current through the transmit coilduring at least a part of the second period of at least a first sequenceto produce a first control signal; controlling at least a part of thetransmit signal during at least one of: a part of the second period ofthe first sequence and a part of the second period of one or moresubsequent sequences using the first control signal such that thereactive transmit voltage across the transmit coil during subsequentsecond periods of the transmit signal is maintained approximatelyconstantly zero with a corresponding current through the transmit coilbeing non-zero; receiving a receive magnetic field using a receive coilto produce a receive signal; processing the receive signal during atleast a part of the second period; and producing an indicative outputsignal indicating the presence of the target based on the processing ofthe receive signal.
 2. A method according to claim 1, wherein therepeating sequence includes a period of high voltage and a period of lowvoltage, the period of high voltage includes the first period, and theperiod of low voltage includes the second period.
 3. A method accordingto claim 1, wherein the controlling at least a part of the transmitsignal includes controlling a magnitude of the voltage of the transmitsignal during the second period.
 4. A method according to claim 1,wherein each repeating sequence further includes a third and a fourthperiod, the third period preceding the fourth period, the method furtherincludes the steps of: sensing a current through the transmit coilduring at least part of the fourth period of a second sequence toproduce a second control signal; controlling at least a part of thetransmit signal during at least one of: the fourth period of the secondsequence and the fourth period of one or more subsequent sequences usingthe second control signal such that the reactive transmit voltage acrossthe transmit coil during the fourth period of the repeating sequence ismaintained approximately constantly zero with a corresponding currentthrough the transmit coil being non-zero, the at least a part of thetransmit signal includes a magnitude of the transmit signal during thefourth period; and wherein the processing includes processing thereceive signal during at least part of the fourth period.
 5. A methodaccording to claim 4, wherein the first sequence and the second sequenceare the same sequence.
 6. A method according to claim 4, wherein thepolarity of an average voltage during the third period is opposite thatof an average voltage during the first period.
 7. A method according toclaim 4, wherein the method further includes the steps of: sensing adifference in an absolute value of a current through the transmit coilduring the second period of a sequence and that of a current through thetransmit coil during the fourth period of the same or a differentsequence; and controlling a ratio of energy transferred to the transmitcoil during the first period and the third period of a subsequentsequence such that the difference in the absolute value of a currentthrough the transmit coil during the second period of the subsequentsequence and that of a current through the transmit coil during thefourth period of the subsequent sequence is approximately zero.
 8. Amethod according to claim 4, wherein a waveform of a current through thetransmit coil is approximately a rectangular wave with a slew rateapproximately proportional to a voltage applied to the transmit coilduring the first period and the voltage applied to the transmit coilduring the third period.
 9. A method according to claim 4, wherein anabsolute value of an average voltage during the third period is lowerthan that of an average voltage during the first period.
 10. A methodaccording to claim 4, wherein the polarity of a current through thetransmit coil changes from one sign to opposite sign during the thirdperiod.
 11. A method according to claim 1, wherein the processing thereceive signal further includes processing the receive signal during atleast a part of a period with non-zero reactive transmit voltage acrossthe transmit coil.
 12. A method according to claim 11, wherein theprocessing of the receive signal during at least a part of the secondperiod includes demodulating or sampling the receive signal to produce aresistive receive signal, the processing of the receive signal during atleast a part of the period with non-zero reactive transmit voltageincludes demodulating or sampling the receive signal to produce areactive signal, the processing of the receive signal further includingthe steps of: differentiating the reactive receive signal to produce adifferentiated receive signal; and combining linearly the differentiatedreceive signal and the resistive signal.
 13. A method according to claim12, wherein timings and gains for demodulating or sampling the receivesignal is selected such that the resistive receive signal isapproximately intrinsically statistically insensitive to signals frommagnetic soils but is responsive to magnetically permeable material rateof change of coupling between the transmit magnetic field and thereceive magnetic field, and the combining linearly is arranged andadapted to approximately cancel a component of the resistive receivesignal which is responsive to magnetically permeable material rate ofchange of coupling between the transmit magnetic field and the receivemagnetic field.
 14. A method according to claim 1, wherein thecontrolling includes producing a constant voltage applied to thetransmit coil during the second period using a sample-and-hold element.15. A method according to claim 1, wherein the controlling includesmaintaining the reactive transmit voltage constant when an inductance ofthe coil is altered by a varying proximity of the coil and the soil. 16.A method accordingly to claim 4, wherein each repeating sequence furtherincludes a fifth period, the fourth period preceding the fifth period,and an absolute value of a current through the transmit coil isincreasing during the fifth period.
 17. A method according to claim 1,wherein the controlling at least a part of the transmit signal includesmaintaining the transmit signal the same during first periods of thetransmit signal.
 18. A method according to claim 1, wherein the transmitsignal is controlled to include more than one receive periods duringwhich the reactive transmit voltage across the transmit coil ismaintained approximately constantly zero with a corresponding currentthrough the transmit coil being non-zero, each of the receive periods iscontrolled by separate control signals, and wherein the processing ofthe receive signal during at least a part of each of the receive periodsincludes demodulating or sampling the receive signal to produceresistive receive signals.
 19. A metal detector configurable to performthe method according to claim 1.